The present invention relates to a hydraulic apparatus for a continuously variable transmission for motorized vehicles, and more particularly to a hydraulic apparatus which controls speed ratios which can be varied.
A continuously variable transmission, having a V-belt extending between driving and driven pulleys, permits control of the number of revolutions of the driven pulley by varying the size of the V-shaped opening in either the driving pulley, the driven pulley or simultaneously varying both of the pulleys. The size of the V-shaped opening is varied by changing the amount of pressure being fed into a hydraulic cylinder of either of the pulleys. Hence, the effective diameter of the pulley can be varied by changing the amount of pressure being fed into the hydraulic cylinder, thereby allowing for a continuously varying ratio of driving pulley diameter to driven pulley diameter. The V-shaped openings on the driving and driven pulleys are each defined by an area between a fixed pulley and a movable pulley, which is dependent upon the pressure inputted to the hydraulic cylinders. In order to provide hydraulic pressure to each of the movable pulleys, a hydraulic apparatus is provided.
A prior hydraulic apparatus was designed in a manner which allowed hydraulic fluid under a constant predetermined pressure to always be supplied to one movable pulley provided in the apparatus. Additionally, a certain amount of the hydraulic fluid supplied to or drained from the another area, permitted control of the V-shaped opening by varying the amount of hydraulic fluid drained from the apparatus. Through this control, the diameter ratio of the pulleys could be continuously varied, resulting in the speed ratio being able to be continuously varied. However, according to this known hydraulic apparatus, a selector valve was used for the hydraulic cylinder of a movable pulley and was subjected to the following three conditions:
The first condition was that the hydraulic fluid, having a constant predetermined amount of pressure, was supplied to the cylinder of a movable pulley. The second condition was that the hydraulic fluid was drained from the movable pulley. The third condition was that the hydraulic fluid was neither supplied to the cylinder of the movable pulley nor drained from the cylinder of the another movable pulley. According to such a known selector valve, the speed ratio was changed at a uniform speed regardless of whether the speed ratio was increased or decreased. Therefore, the change of the actual speed ratio by the aforementioned selector valve could not promptly follow the required speed ratio needed in order to satisfy specific driving conditions. For example, the responsive change of the speed ratio was not fast enough to meet the changing driving conditions, which resulted in a time delay between the point at which a ratio change was required and when the ratio change actually occurred.